zero-day
adjective
ze·ro-day
ˈzir-(ˌ)ō-ˈdā 
ˈzē-(ˌ)rō-
computing
: of, relating to, or being a vulnerability (as in a computer or computer system) that is discovered and exploited (as by cybercriminals) before it is known to or addressed by the maker or vendor
Thus, in the parlance of the trade, these vulnerabilities are known as "zero-day exploits," because it has been zero days since they have been uncovered and fixed.—
James Bamford
James BamfordThere's a zero-day vulnerability in RealPlayer and Internet Explorer that allows malicious code to be installed …—Mark Hopkins
Mark Hopkins… most of what's on the black market doesn't have that zero-day freshness. Criminals typically target older vulnerabilities, for which fixes have already been released …—Lev Grossman
Lev GrossmanExamples of zero-day in a Sentence
Recent Examples on the Web
Previous Pwn2Own events have seen the Samsung Galaxy S25 exploited by the zero-day hackers, and Meta offer a staggering $1 million reward for a WhatsApp 0-click exploit.
—Davey Winder, Forbes.com, 15 May 2026
The company called it a zero-day exploit, a cyberattack that takes advantage of a previously unknown security vulnerability.
—Matt O'Brien, Fortune, 11 May 2026
Shortly after the disclosure, someone else leaked key details, effectively making the vulnerability a zero-day.
—Dan Goodin, ArsTechnica, 11 May 2026
Think of it as a zero-day exploit.
—Aidan McLaughlin, Vanity Fair, 16 Apr. 2026
See All Example Sentences for zero-day
Examples are automatically compiled from online sources to
show current usage.
Read More
Opinions expressed in the examples do not represent those of Merriam-Webster or its editors.
Send us feedback.
Davey Winder, Forbes.com, 15 May 2026
The company called it a zero-day exploit, a cyberattack that takes advantage of a previously unknown security vulnerability.
—Matt O'Brien, Fortune, 11 May 2026
Shortly after the disclosure, someone else leaked key details, effectively making the vulnerability a zero-day.
—Dan Goodin, ArsTechnica, 11 May 2026
Think of it as a zero-day exploit.
—Aidan McLaughlin, Vanity Fair, 16 Apr. 2026
See All Example Sentences for zero-day
Cite this Entry
“Zero-day.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/zero-day. Accessed 20 May. 2026.
Copy CitationLove words? Need even more definitions?
Merriam-Webster unabridged
